After the sprinkling and concluding prayers, the priest goes back to the sacristy, changes his cope for the chasuble while thurifer, crucifer, acolythi, and so forth are lining up at the back of church. During this time, the organist improvises on the Introit. As the priest enters—this time from the back of church—the Introit is sung.
Here’s a special ASPERGES ME score you’ll want to download immediately:
Also, download audio recordings and organ accompaniments at the Saint Antoine Daniel Kyriale.